<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>

<body>
    <style>
        section{
            width: 400px;
            height: 400px;
            background-color: blueviolet;
            margin: 300px;
            position: relative;
        }
        div {
            width: 100px;
            height: 100px;
            border:solid 5px #ddd;
            padding: 10px;
            box-sizing: border-box;
            background-color: burlywood;
            background-clip: content-box;
        }
        div:nth-child(2){
            position: absolute;
            top:0;
            z-index:-1;
        }
        div:nth-child(3){
            position: fixed;
            bottom:-50px;
            left:0;
            right:0;
            width:auto;
            z-index:3;
        }
    </style>
     <!-- z-index:0 
     普通元素没有层级概念:null-->
<section>
    <div>1</div>
    <div>2</div>
    <div>3</div>
</section>
</body>

</html>